1. Field of the Invention
The present invention relates to a method for supplying CZ raw material which is used in forming the raw material melt in a single crystal growth according to the CZ method, and in particular to a method for supplying CZ raw material for use in additional charging of poly-silicon raw material.
2. Description of the Related Art
As is well known, in the single crystal growth according to the CZ method, the solid poly-silicon raw material initially charged into a crucible is melted by heating with a heater. Once the raw material melt is thus formed in the crucible, a seed crystal is immersed into the raw material melt in the crucible, and then a cylindrical single crystal is grown under the seed crystal by pulling the seed crystal while the seed crystal and the crucible are rotated. Poly-silicon cut rods, chunks, grains, and the like are used independently or in combinations thereof as solid poly-silicon raw material to be charged into the crucible initially.
In such single crystal growth, when the solid raw material initially charged into the crucible is melted, the bulk is decreased and the volume of the raw material melt is decreased compared to the crucible volume, unavoidably leading to a decrease of the productivity. In order to avoid this fault, the amount of the raw material to be charged into the crucible is considered to be increased, and as such a remedy there has been developed a technique referred to as additional charge.
In the additional charge, after the solid raw material initially charged into the crucible has been melted, solid poly-silicon raw material is charged additionally into the raw material melt in the crucible. As such a mode of raw material charging, there is a technique wherein a cylindrical mass of poly-silicon raw material, hung over the crucible with a wire to be used for crystal pulling, is gradually lowered and immersed into the raw material melt in the crucible. Accordingly, the volume of the raw material melt in the crucible is increased, and the volume of the crucible is effectively utilized, improving the productivity.
In such prior additional charge techniques, however, while a cylindrical mass of poly-silicon raw material is immersed in the raw material melt in the crucible, an axial and a radial thermal strain are caused to occur in the poly-silicon raw material, and thereby cracks are generated during the immersing. As a result, rupture takes place frequently in the axially intermediate portion, and at a time a large amount of poly-silicon raw material drops into the raw material melt in the crucible. When such dropping occurs, the crucible may be broken and there may be a high possibility of the melt run-out hazard that a large amount of the raw material melt flows into the furnace.
In another additional charge technique, by use of a raw material supplying pipe inserted into the furnace, granules/lumps poly-silicon raw material is dropped into the raw material melt in the crucible; when the raw material is dropped in, there may occur troubles such as the splashing of the melt droplets, the bouncing of the raw material itself, and furthermore the damaging of the crucible caused by the collision of the raw material being dropped in, and the like. Incidentally, as for the raw material, the cost of granules/lumps raw material is lower than that of cylindrical mass of raw material.
The object of the present invention is to provide a method for supplying CZ raw material which method allows additional charging of granules/lumps poly-silicon raw material, low in raw material cost and unsusceptible to crack, into a crucible in a static and steady manner.
In order to achieve the above-mentioned object in growing single crystals according to the CZ method, in the method for supplying CZ raw material of the present invention, a tubular container charged with lumps and/or granules poly-silicon raw material is placed above the poly-silicon raw material initially charged into the crucible, and the poly-silicon raw material in the tubular container is additionally fed to the crucible as the melting of the poly-silicon raw material in the crucible proceeds.
According to the present invention, as the bulk of poly-silicon raw material in the crucible is decreased in parallel with the melting of the poly-silicon raw material in the crucible, the poly-silicon raw material in the tubular container gradually and spontaneously descends by its own weight to get into the crucible. Thus, the poly-silicon granules/lumps raw material unsusceptible to crack is additionally charged into the crucible in a static and steady manner without being accompanied by the splashing of the melt as well as the bouncing of the raw material.
The tubular container is made of a heat-resistant material, which material preferably comprises particularly carbon unsusceptible to heavy-metal contamination, carbon material at least with its interior surface coated with SiC, or SiC. Only with carbon, while the poly-silicon raw material in the tubular container moves downward, the interior surface of the tubular container may possibly be scraped off, resulting in contamination of the melt with carbon, whereas the SiC coating prevents such possible contamination.
Preferably, the tubular container has such a partitioned structure that the tubular container can be partitioned circumferentially into a plurality of parts, and the parts are combined so as to be movable outward. Thus, the container can radially expand according to the thermal expansion of the poly-silicon raw material in the container, so that the clogging of the poly-silicon raw material is prevented effectively.